Mortality following surgery is caused primarily by complications of infection, but limited understanding of pathogenesis has hampered the development of new therapeutics. Endotoxin produced by all Gram- negative bacteria is lethal to mammals because it stimulates macrophages to release TNF and other potentially lethal factors. Maximal TNF levels are achieved within two hours after dosing with endotoxin; this timing is critical because even minimal delays in administering treatment against early mediators renders them ineffective. Paradoxically, endotoxin- poisoned animals succumb at latencies up to five days after the onset of endotoxemia, long after serum levels of the early cytokines TNF and IL-1 return to basal levels. Dr. Tracey and his colleagues recently identified a previously unrecognized "late" mediator of endotoxin lethality, a ubiquitous protein known as "high mobility group-1" (HMG-1) (Science, 1999, 285: 248-251). Endotoxin stimulates LPS-sensitive macrophages to release HMG-1; LPS-resistant macrophages from C3H/HeJ mice fail to release HMG-1. Serum HMG-1 levels are significantly increased from 16 to 32 hours after endotoxemia in mice. In serum from patients with surgical sepsis, high HMG-1 levels were associated with lethal outcome. Administration of rHMG-1 to mice is lethal, and delayed administration of anti-HMG-1 antibodies significantly protects against lethal endotoxemia, even when antibody administration is delayed until after peak TNF release. To date however, the effects of anti-HMG-1 or HMG- 1 itself in a clinically relevant animal model of trauma, injury, or hemorrhagic shock are unknown. Trauma and shock are known to induce a state of altered cytokine responsiveness in which macrophage activation states are deranged. It is theoretically possible that anti-HMG- 1 antibodies used in this setting would not improve survival; they may actually be deleterious. The objective of the studies outlined in this proposal is to determine whether HMG-1 mediates beneficial or injurious responses in the altered cytokine milieu of posttraumatic sepsis. The experiments proposed in the Specific Aims will define the effects of HMG-1 and antiHMG-1 antibodies in a clinically relevant model of sepsis in animals subjected to surgery and hemorrhagic shock. Studies of the binding and uptake of HMG-1 in human and murine monocytes, and the stimulation of cytokine release by HMG-1 itself, will yield important new information concerning the endogenous mechanisms underlying the systemic response to shock and sepsis.